1. Field of the Invention
This invention generally relates to Multiple-Input Multiple-Output (MIMO) Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier Frequency Division Multiple Access (SC-FDMA) systems, and more particularly, to channel estimation for MU-MIMO systems where the different users' reference signals are multiplexed by applying orthogonal cyclic shifts.
2. Description of the Related Art
FIG. 1 is a diagram depicting a Single-User MIMO (SU-MIMO) wireless communication system (prior art). Multiple transmit (Tx) and receive (Rx) antennas are used to send multiple data streams in parallel at the same frequency. MIMO processing increases the data rates by up to a factor of min(Mt, Mr), as compared to single-antenna systems. Each receive antenna collects a linear combination of all Tx antenna signals, resulting in inter-stream interference. There is a need to decouple the spatial streams at the receiver. This decoupling is accomplished via MIMO equalization. MIMO equalization requires estimates of the channel matrix H, which is the gain and phase response between each Tx and Rx antenna pair at a particular time and frequency.
As noted in Wikipedia, MIMO can be sub-divided into three main categories, precoding, spatial multiplexing (SM), and diversity coding. Precoding is multi-layer beamforming in a narrow sense or all spatial processing at the transmitter in a wide-sense. In (single-layer) beamforming, the same signal is emitted from each of the transmit antennas with appropriate phase (and sometimes gain) weighting such that the signal power is maximized at the receiver input. The benefits of beamforming are to increase the signal gain from constructive combining and to reduce the multipath fading effect. In the absence of scattering, beamforming results in a well defined directional pattern, but in typical cellular conventional beams are not a good analogy. When the receiver has multiple antennas, the transmit beamforming cannot simultaneously maximize the signal level at all of the receive antennas, and precoding is used. Note that precoding requires knowledge of the channel state information (CSI) at the transmitter.
Spatial multiplexing requires MIMO antenna configuration. In spatial multiplexing, a high rate signal is split into multiple lower rate streams and each stream is transmitted from a different transmit antenna in the same frequency channel. If these signals arrive at the receiver antenna array with sufficiently different spatial signatures, the receiver can separate these streams, creating parallel channels free. Spatial multiplexing is a very powerful technique for increasing channel capacity at higher Signal to Noise Ratio (SNR). The maximum number of spatial streams is limited by the lesser in the number of antennas at the transmitter or receiver. Spatial multiplexing can be used with or without transmit channel knowledge.
Diversity Coding techniques are used when there is no channel knowledge at the transmitter. In diversity methods a single stream (unlike multiple streams in spatial multiplexing) is transmitted, but the signal is coded using techniques called space-time coding. The signal is emitted from each of the transmit antennas using certain principles of full or near orthogonal coding. Diversity exploits the independent fading in the multiple antenna links to enhance signal diversity. Because there is no channel knowledge, there is no beamforming or array gain from diversity coding. Spatial multiplexing can also be combined with precoding when the channel is known at the transmitter or combined with diversity coding when decoding reliability is in trade-off.
FIG. 2 is a diagram depicting a Multiuser MIMO (MU-MIMO) wireless communication system (prior art). Multiple users can transmit data simultaneously at the same frequency to a multi-antenna base station, resulting in increased aggregate cell throughput. There is a need to decouple data streams from different users via MU-MIMO equalization, which requires MU-MIMO channel estimation.
FIG. 3 is a diagram depicting a subframe consisting of two slots, as is used in Long Term Evolution (LTE) (prior art). LTE is the Third Generation Partnership Program (3GPP) term for the next generation cellular standard. The figure shows two resource blocks, with one resource block per slot. Each slot includes seven OFDMA or SC-FDMA symbols for normal CP, or 6 symbols for extended CP, at twelve subcarrier frequencies. In OFDMA and SC-FDMA, each user is allocated resource elements (REs) in time and frequency. SC-FDMA is similar to OFDMA except that user data are spread via a discrete Fourier transform (DFT) before OFDMA modulation. Each resource element consists of 1 subcarrier in the frequency domain and 1 OFDMA or SC-FDMA symbol in the time domain. User data modulates the amplitude and phase of each subcarrier for the duration of 1 OFDMA or SC-FDMA symbol. Multiple users can modulate the same RE (MU-MIMO). In the LTE uplink, each user transmits reference signals on all REs of specified symbols. Different user reference signals are multiplexed using different cyclic shifts. The base station uses the reference signals to estimate a channel for each user.
FIG. 4 is a diagram depicting an exemplary OFDMA frequency spectrum (prior art). OFDMA is a multi-user version of the popular Orthogonal frequency-division multiplexing (OFDM) digital modulation scheme. Multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual users as shown. This allows simultaneous low data rate transmission from several users. OFDMA is recognized as being highly sensitive to frequency offsets and phase noise. OFDMA can also be described as a combination of frequency domain and time domain multiple access, where the resources are partitioned in the time-frequency space, and slots are assigned along the OFDM symbol index as well as OFDM sub-carrier index. OFDMA is considered as highly suitable for broadband wireless networks, due to advantages including scalability and MIMO-friendliness, and ability to take advantage of channel frequency selectivity.
SC-FDMA is a multi-user version of Single-carrier frequency-domain-equalization (SC-FDE) modulation scheme. SC-FDE can be viewed as a linearly precoded OFDM scheme, and SC-FDMA can be viewed as a linearly precoded OFDMA scheme, henceforth LP-OFDMA. FDE is the equalizer at receiver end. It is different from the modulation scheme. Or, it can be viewed as a single carrier multiple access scheme. Just like in OFDM, guard intervals with cyclic repetition are introduced between blocks of symbols in view to efficiently eliminate time spreading (caused by multi-path propagation) among the blocks. In OFDM, a Fast Fourier transform (FFT) is applied on the receiver side on each block of symbols, and inverse FFT (IFFT) on the transmitter side. In SC-FDMA, both FFT and IFFT are applied on the receiver side, but not on the transmitter side. In SC-FDMA, both FFT and IFFT are applied on the transmitter side, and also on the receiver side.
In OFDM as well as SC-FDE and SC-FDMA, equalization is achieved on the receiver side after the FFT calculation, by multiplying each Fourier coefficient by a complex number. Thus, frequency-selective fading and phase distortion can be combated. The advantage is that FFT and frequency domain equalization requires less computation power than conventional time-domain equalization. In SC-FDMA, multiple access is made possible by inserting zero Fourier-coefficients on the transmitter side before the IFFT, and removing them on the receiver side after the FFT. Different users are assigned to different Fourier-coefficients (subcarriers).
LTE uses OFDMA for the downlink—that is, from the base station to the terminal. In the time domain the radio frame is 10 ms long and consists of 10 sub frames of 1 ms each. In LTE with frequency-division duplexing (FDD), every sub frame consists of 2 slots where each slot is 0.5 ms. The subcarrier spacing in the frequency domain is 15 kHz and there are modes with 7.5 kHz subcarrier spacing. Twelve of these subcarriers together (per slot) are called a resource block, so one resource block is 180 kHz. 6 Resource blocks fit in a carrier of 1.4 MHz and 100 resource blocks fit in a carrier of 20 MHz. In the uplink, for the Physical Uplink Shared channel (PUSCH) only, LTE uses a pre-coded version of OFDMA called SC-FDMA to compensate for a drawback with normal OFDMA, which has a very high peak-to-average power ratio (PAPR). High PAPR requires expensive and inefficient power amplifiers with high requirements on linearity, which increases the cost of the terminal and drains the battery faster. SC-FDMA solves this problem by grouping together the resource blocks in a way that reduces the need for linearity, and so power consumption, in the power amplifier. A low PAPR also improves coverage and the cell-edge performance.
In MIMO systems, a transmitter sends multiple streams by multiple transmit antennas. The transmit streams go through a matrix channel which consists of all paths between the transmit antennas at the transmitter and receive antennas at the receiver. Then, the receiver gets the received signal vectors by the multiple receive antennas and decodes the received signal vectors into the original information. A narrowband flat fading MIMO system is modeled as:y=Hx+n
where y and x are the receive and transmit vectors, respectively, and H and n are the channel matrix and the noise vector, respectively. Where x is a Mt×1 vector, y and n are Mr×1 vectors.
With respect to MU-MIMO channel estimation for OFDMA/SC-FDMA, user reference signals with different cyclic shifts are orthogonal across a number of tones in ideal scenarios (no timing offset and low delay spread). In this case, channel estimation for each user is decoupled. Several channel estimation techniques exist in prior art, such as least squares, minimum mean-square error (MMSE), discrete cosine transform (DCT), can be used under the orthogonality assumption. In practice, orthogonality is destroyed because of different user timing offsets and/or medium to high delay spreads.
FIG. 5 is a diagram depicting an exemplary MIMO receiver (prior art). Channel estimation is needed in multi-user and single-user MIMO receivers to separate different spatial streams and/or user signals via equalization. Of special interest is OFDMA and SC-FDMA multi-user MIMO channel estimation with a single spatial stream per user (e.g., LTE uplink). The input to the channel estimator block is the received frequency domain signal of reference symbols from Mr receive antennas. The outputs are channel responses in the frequency domain from user u (1≦u≦U) to antenna m (0≦m≦Mr−1).
FIG. 6 is a drawing depicting uplink reference signals in LTE (normal cyclic prefix) (prior art). The reference signals of the different users are orthogonal across a number of tones if the same base sequence is used and each user applies a unique cyclic shift. The orthogonality holds in the additive white Gaussian noise (AWGN) channel with no relative timing offset across users, but not in a multipath fading channel or with non-zero relative timing offsets across users. This is a problem because the user timing offsets are unknown to the receiver.
In signal processing, direction of arrival denotes the direction from which usually a propagating wave arrives at a point, where usually a set of sensors are located. This set of sensors forms what is called a sensor array. Often there is the associated technique of beamforming which is estimating the signal from a given direction. With beamforming, spatial selectivity is achieved by using adaptive or fixed receive/transmit beampatterns. The improvement compared with an omnidirectional reception/transmission is known as the receive/transmit gain (or loss). Beamforming takes advantage of interference to change the directionality of the array. When transmitting, a beamformer controls the phase and relative amplitude of the signal at each transmitter, in order to create a pattern of constructive and destructive interference in the wavefront. When receiving, information from different sensors is combined in such a way that the expected pattern of radiation is preferentially observed. With narrow-band systems the time delay is equivalent to a “phase shift”, so in this case the array of antennas, each one shifted a slightly different amount, is called a phased array. A narrow band system is one where the bandwidth is only a small fraction of the centre frequency.
It would be advantageous if an approach existed for MU-MIMO channel estimation for OFDMA/SC-FDMA that accounted for non-orthogonal user reference signals.